Tigers are hard to count. Because tigers live in dense forests, they are hard to find, and counting them from the air or from vehicles does not work well. The size of a tiger's range depends on how much prey there is. Female Indian tigers have home ranges of 200 to 1000 square kilometers, and a male's home range averages 2 to 15 times larger. Tigers mostly live alone, and they mark their range with a musky scent sprayed on grass, trees and rocks, and with scrapes and claw marks. Earlier, India estimated its tigers with track counts that identified individual tigers by their pug marks (footprints). This failed to give reliable estimates because of erroneous assumptions. Most early methods of counting tigers failed because they did not take three things about tigers into account: tigers are scarce, their range is huge, and they are secretive.
Cameras and stripes changed the work. In the 1920s a British forester in India, F.W. Champion, used cameras with tripwires to photograph tigers and leopards in the foothills of the Western Himalayas. Scientists use motion-sensitive cameras in forests that let tigers take their own pictures. No two tigers have the same stripes, and scientists can tell individual wild tigers apart by their stripe patterns. A study in India photographed tigers with camera traps, compared stripe shape and position between pictures, and used the program CAPTURE to compute tiger abundance. Matching thousands of tiger pictures by eye is hard, so a pattern-matching program called ExtractCompare, developed by statistician Lex Hiby, extracts stripe patterns and shortlists the closest matches. Trained technicians then pick the tiger from the shortlist.
Counting is a big project. One research team in India reported in 2015 that each year it set up over 700 camera-trap stations covering 6,000 square kilometers, which produced over 15,000 images of tigers, leopards, dholes, elephants, deer and other animals. By 2015 one team's two decades of camera-trap pictures had identified almost 800 individual tigers by their stripes. In Bangladesh, Smithsonian scientists collect field data on tigers, prey and mangrove habitat, including with camera-trap studies to estimate tiger populations. In 2008 the Smithsonian joined the World Bank Group and the Global Environmental Facility to launch the Global Tiger Initiative. Panthera says its Tigers Forever program monitors tigers and their prey in critical sites across five tiger range countries.
Counts have improved, and here are the dated totals. As recently as 15 years before 2022, scientists made educated guesses about tiger numbers; camera traps, genetic testing and data modeling have since improved counting. A 2022 IUCN assessment estimated between 3,726 and 5,578 wild tigers in Asia, with an average of 4,500. In that assessment, 3,140 of the 4,500 estimated wild tigers were adults. Tigers once lived across much of Asia. As of July 2022, tigers had lost 93% of their historic range, and their numbers went from 100,000 in the last century to potentially 4,500 in 2022.
Counts show where tigers are found. After counting tigers in four Indian reserves, researchers concluded that moist deciduous forests and tropical alluvial grasslands were the best habitats for tigers to reach their highest densities. Tigers are known to live in tropical lowland evergreen forest, monsoonal forest, dry thorn forest, scrub oak and birch woodlands, tall grass jungles and mangrove swamps. Tigers have been found as high as 3,960 meters above sea level. In general they need only some plant cover, a source of water and enough prey. Tigers can live in warm, wet forests and in places with heavy snow where temperatures may fall as low as -40 degrees Celsius.
Behind each number is a tiger growing up. Newborn tiger cubs are blind and weigh 780 to 1,600 grams, and their ears open at 9 to 11 days. A mother tiger begins teaching her cubs to hunt at about 3 months, and the cubs begin to take part in hunting trips at 5 to 6 months old. Tiger cubs stay with their mother for about two years. Tigers keep several dens within their home range, often in dense plants, a cave or a hollow tree. Young female tigers often settle near their mother's territory, while young males disperse farther from home.
Words to know
- erroneous: wrong
- scarce: few in number
- abundance: how many
- IUCN: wildlife rating group
- territory: an animal's area
- disperse: spread out
